Jellyfish help mix the world's oceans

Marine creatures could stir up seas as much as do winds and tides.

Roberta Kwok

Animals such as jellyfish could play a big role in ocean circulation.Nature

Small sea creatures such as jellyfish may contribute to ocean mixing by pulling water along as they swim, according to a new study. The collective movement of animals could generate stirring of the same order as winds and tides, the authors suggest.

Some scientists are sceptical that the process plays a substantial role in mixing the world's oceans. But if borne out by more evidence, the finding could affect models that incorporate ocean-mixing into simulations of past and future climate.

"It's a very intriguing mechanism," says ocean physicist Eric Kunze of the University of Victoria in Canada, who was not involved in the work. Until scientists gather more data, though, "we don't even know whether this process is important in the ocean", he says.

Past studies suggested that small, abundant animals such as krill might help push nutrient-rich water towards the surface by creating turbulence in their wake1. But critics argued that these eddies would quickly peter out2.

Set adrift

Now fluid mechanics researcher John Dabiri and graduate student Kakani Katija at the California Institute of Technology in Pasadena have investigated a different mechanism, first reported by evolutionary biologist Charles Darwin's grandson in 19533. When an object moves through a fluid, he suggested, it pulls some with it.

The two modelled Darwin's mechanism for a range of viscosities, similar to that encountered by animals in ocean water, and found that an object carries along more fluid with it as the viscosity increases. Objects the size of small marine plankton pulled up to four times their volume in fluid by moving just a few body lengths, Dabiri says.

“We need to look at animals as not just responding to the ocean, but actively driving it in certain places.”

John Dabiri California Institute of Technology

The researchers also monitored jellyfish as they swam through clouds of dye in a lake on the Pacific island of Palau. A trail of dye followed each animal, as Darwin's mechanism would predict (see video). Using a laser-equipped camera, the team then measured the dye's movement and the stirring of suspended particles in the animal's wake. Ninety per cent of the water's mixing came from Darwin's mechanism, rather than from wake turbulence, the team reports in Nature4.

Darwin's mechanism could produce about 1 trillion watts of power in the world's oceans, the same order of magnitude as winds and tides, the team estimates. "We need to look at animals as not just responding to the ocean, but actively driving it in certain places," says Dabiri.

Climate complications?

But others are yet to be convinced. Since the water being pulled upward is denser, it will eventually drop away rather than travel with the animal, argues André Visser, an oceanographer at the Technical University of Denmark in Charlottenlund. "It's far from convincing, at least in my mind," he says.

Dabiri agrees that water may resettle quickly in some locations but says this falling process will still generate mixing. If animals are travelling in a dense group, he says, fluid drifting down from one animal may be picked up by a neighbour.

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